Reference Variable Sheet in macro

I have used coding similar to the following before:

 ActiveCell.FormulaR1C1 = "=Sheet1!R[3]C[3]"

However, in this instance, my sheet is a variable (strSheet) and I can't quite get the syntax right. So far, I have tried the following to no avail:

ActiveCell.FormulaR1C1 = "=strSheet!R[3]C[3]"
ActiveCell.FormulaR1C1 = "=&strSheet&!R[3]C[3]"

Any help would be greatly appreciated!